I've perused the archives but not found any ready info on DC powered amps (granted I haven't spent a ton of time looking, but I have two young children...). I recall something about 12V amps for cars, but I didn't find much about them. I am hoping in the next 5 years or so to begin to switch to solar power at home (mix of active and passive) and am interested in DC powered gear. In particular, designs for amps (tube preference) to run directly from 12 volt.

So...a few questions. First, any suggestions for good 12V designs, or at least so ideas to look at? Second, do they require inversion to AC, or can DC power an AC (e.g audio) signal without inversion to AC? What kinds of things should I know and/or be concerned about?

most have a converter for mains supplies to get to DC. For cars, there is another type of converter to go from low voltage DC to (very high frequency AC) what looks like higher voltage DC.

Do some reading on switching power supplies.

For high power amplifiers, you will need to convert the DC to a higher level. For low power amplifiers (solid state) you can run them straight off 12V. For tubes, you will need a step up in voltage. If you are running a house off batteries, you will likely be wanting to save electricity, and tubes are (usually) not the way to do that. Class D, H, "T" switching amplifers will be the way to go to save energy.

The Laws of Physics are against you using 12 V. in a power amp. There are several equations for power. The equation of interest is: P = V^2/R. The max. theoretical power a 12 V. rail can produce in an 8 Ohm load is 18 W. In the real world, it's a good deal less. High powered auto amps have switching power supplies in them to get the necessary Volts.

Another preamp tube that works well with a low B+ rail is the 6GM8.

If you use 45 V. for B+, there is no serious shock hazard to your kids. You could construct something that uses push/pull 3S4s as the O/P devices. HIGHLY efficient horn speakers would be necessary, as we are talking about 700 mW. from a PP pair of 3S4s.

You need to get your facts and theory correct. How can a 12v supply deliver 18 watts to an 8 ohm load? In order to deliver 18 watts into 8 ohms you must deliver 12v RMS in AC volts.

So if you power supply is 12 volts DC, this means that the peak to peak value of the supply is 12 volts. If an amplifier output stage is connected across the 12 volt supply, the centre rail (or speaker rail) can only swing 6v positive peak and 6v negative peak.....that's all no more no less (Ignoring the inevitable losses in the output devices for the moment).

So if the signal can only swing 6v peak (or 12v pk-pk) the RMS voltage of 6 volts peak is 4.2426 volts RMS. Now square this and divide by 8 and the answer is------2.249 watts, a far cry from your "18 watts".

Even if you bridge a pair of amplifiers you can the deliver 8,48 volts and this will give you 8.98 watts.

Add in losses in the output stage (typically you will drop about 0.3 volts if you saturate your output devices hard so actaul power is even less.

This is why these car audio hewad unit companies who claim their CD/FM/AM receivers can deliver 50 watts peak from a 12 volts supplyare ALL lying! Even if you give them the benefit of the battery going to 14.4v a pair of bridged amplifiers with a 14.4 volt supply can only deliver 14.4v /2 =7.2 v pk. Now x 0.7071 gives 5.09 volts RMS per half bridge. The bridge pair will deliver 10.1 volts RMS to the load. Load this with 4 ohms and the saturation drop of the output devices will be typically 0.8-1 volt. So you end up with at best 9 volts RMS across your load. At 4 ohms you get 20.25 watts and 8 ohms 10.125 watts.

I gave things in a simplistic DC situation. You are, of course, correct about AC RMS considerations. Please notice that I did state that real world conditions (including AC RMS) would greatly reduce the power available.

Carl may not have to deal with diode drops. Remember, he's discussing a DC photovoltaic situation.

Bridging definitely helps the available power cause. Max. AC power with a 12 VDC rail into 4 Ohms is 18 W. A bridged pair of sections can produce 72 W. into a 4 Ohm load, as each section "sees" a 2 Ohm load. That's both a blessing and a curse, as each of the amp sections has to be stable into the 2 Ohm load.

Wow, excellent discussion all. Thank you. I know the 12V limit is problematic, but on a PV system, I don't care to put out a ton of power. I just built my first SET that puts out only about 1 watt/channel, and it's fine for me for a tube listening situation, even on an old pair of Boston Acoustics A40s at 88dB efficiency (if I want noise, I can power up my ss Luxman - for tubes, I'm interested in sitting down and really listening!).

Now, any suggestions for further reading, and any designs I should look at? I'm not yet at the point where I could design a 12V amp from the ground up. Eli, I like the 45V B+, especially with 2 young children in the house, so if you have references for where to go with that idea, I'd appreciate it. Or should I just look at auto tube designs?